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The Single UNIX &reg; Specification, Version 2<br>
Copyright &copy; 1997 The Open Group

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<h4><a name = "tag_000_008_570">&nbsp;</a>NAME</h4><blockquote>
shmget - get shared memory segment
</blockquote><h4><a name = "tag_000_008_571">&nbsp;</a>SYNOPSIS</h4><blockquote>
<pre><code>

#include &lt;<a href="sysshm.h.html">sys/shm.h</a>&gt;

int shmget(key_t <i>key</i>, size_t <i>size</i>, int <i>shmflg</i>);
</code>
</pre>
</blockquote><h4><a name = "tag_000_008_572">&nbsp;</a>DESCRIPTION</h4><blockquote>
The
<i>shmget()</i>
function returns the shared memory identifier associated with
<i>key</i>.
<p>
A shared memory identifier, associated data structure
and shared memory segment of at least
<i>size</i>
bytes, see
<i><a href="sysshm.h.html">&lt;sys/shm.h&gt;</a></i>,
are created for
<i>key</i>
if one of the following is true:
<ul>
<p>
<li>
The argument
<i>key</i>
is equal to IPC_PRIVATE.
<p>
<li>
The argument
<i>key</i>
does not already have a shared memory identifier
associated with it and (<i>shmflg</i>&amp;IPC_CREAT) is non-zero.
<p>
</ul>
<p>
Upon creation, the data structure associated with
the new shared memory identifier is initialised as follows:
<ul>
<p>
<li>
The values of
<i>shm_perm.cuid</i>,
<i>shm_perm.uid</i>,
<i>shm_perm.cgid</i>
and
<i>shm_perm.gid</i>
are set equal to the effective user ID and
effective group ID, respectively, of the calling process.
<p>
<li>
The low-order nine bits of
<i>shm_perm.mode</i>
are set equal to the low-order nine bits of
<i>shmflg</i>.
The value of
<i>shm_segsz</i>
is set equal to the value of
<i>size</i>.
<p>
<li>
The values of
<i>shm_lpid</i>,
<i>shm_nattch</i>,
<i>shm_atime</i>
and
<i>shm_dtime</i>
are set equal to 0.
<p>
<li>
The value of
<i>shm_ctime</i>
is set equal to the current time.
<p>
</ul>
<p>
When the shared memory segment is created, it will be initialised with
all zero values.
</blockquote><h4><a name = "tag_000_008_573">&nbsp;</a>RETURN VALUE</h4><blockquote>
Upon successful completion,
<i>shmget()</i>
returns a non-negative integer,
namely a shared memory identifier;
otherwise, it returns -1 and
<i>errno</i>
will be set to indicate the error.
</blockquote><h4><a name = "tag_000_008_574">&nbsp;</a>ERRORS</h4><blockquote>
The
<i>shmget()</i>
function will fail if:
<dl compact>

<dt>[EACCES]<dd>
A shared memory identifier exists for
<i>key</i>
but operation permission as specified
by the low-order nine bits of
<i>shmflg</i>
would not be granted.
See
<a href="ipc.html">IPC</a>.

<dt>[EEXIST]<dd>
A shared memory identifier exists for the argument
<i>key</i>
but (<i>shmflg</i>&amp;IPC_CREAT)&amp;&amp;(<i>shmflg</i>&amp;IPC_EXCL)
is non-zero.


<dt>[EINVAL]<dd>
The value of
<i>size</i>
is less than the system-imposed minimum or
greater than the system-imposed maximum,
or a shared memory identifier exists for the argument
<i>key</i>
but the size of the segment associated with it is less than
<i>size</i>
and
<i>size</i>
is not 0.

<dt>[ENOENT]<dd>
A shared memory identifier does not exist for the argument
<i>key</i>
and (<i>shmflg</i>&amp;IPC_CREAT) is 0.

<dt>[ENOMEM]<dd>
A shared memory identifier and associated shared memory segment are to be
created but the amount of available physical memory is not sufficient to
fill the request.

<dt>[ENOSPC]<dd>
A shared memory identifier is to be created but
the system-imposed limit on the maximum number of
allowed shared memory identifiers system-wide would be exceeded.

</dl>
</blockquote><h4><a name = "tag_000_008_575">&nbsp;</a>EXAMPLES</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_008_576">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The POSIX Realtime Extension defines alternative interfaces for interprocess communication.
Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
<a href="ipc.html">IPC</a>
can be easily modified to use the alternative interfaces.
</blockquote><h4><a name = "tag_000_008_577">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_008_578">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="shmat.html">shmat()</a></i>,
<i><a href="shmctl.html">shmctl()</a></i>,
<i><a href="shmdt.html">shmdt()</a></i>,
<i><a href="shm_open.html">shm_open()</a></i>,
<i><a href="shm_unlink.html">shm_unlink()</a></i>,
<i><a href="sysshm.h.html">&lt;sys/shm.h&gt;</a></i>,
<a href="ipc.html">IPC</a>.
</blockquote><h4>DERIVATION</h4><blockquote>
Derived from Issue 2 of the SVID.
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Copyright &copy; 1997 The Open Group
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